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Dry summer may threaten foliage in some northcentral counties

September 30, 2008

Dry summer may threaten foliage in some northcentral counties

University Park, Pa. — A perusal of Pennsylvania weather statistics for 2008 so far will show that the year has been pretty average in terms of precipitation, temperatures and number of storms. But figures can be misleading, and in the case of the upcoming fall foliage display, they don't tell the whole story, according to experts in Penn State's College of Agricultural Sciences.

"We have seen a drought in the northcentral part of the state in the past few months," said Marc Abrams, professor of forest ecology and physiology. "Trees in a few counties were getting really stressed, and there were early signs of leaf coloration, wilting, browning and so forth. Recent rains may have eased this situation, but time will tell if they came soon enough."

In dry years, Pennsylvania weather forecasters look to the remnants of hurricanes and tropical storms to break the grip of droughts. This summer is no exception. Hurricane Hannah, which came ashore in early September near the border of North Carolina and South Carolina, eventually dumped up to 6 inches of rain on parts of eastern Pennsylvania. Problem was, its effects weren't felt far enough to the west.

"For the year, precipitation had been slightly below normal for the eastern and central part of Pennsylvania, and western Pennsylvania had been normal," said Bryan Swistock, water resources specialist with Penn State Cooperative Extension. "That was before the remnants of Hannah came through. The eastern part of the state got hammered with rain, which brought the region up to an average level of precipitation for the year.

"The northcentral part of the state needed the rain badly, but it didn't get much of it. The driest counties are Clinton, Clearfield, Cameron, Elk and Potter."

The very dry summer in the affected counties could be bad news for their fall foliage, Abrams worries, but the situation could turn around with more prolonged rains -- if they come soon -- perhaps from another tropical storm. "But some of our very important color-producing species -- red maple, black gum and the birches -- are starting to turn early in those counties," he said. "This is a classic drought response. So we may see a less defined peak in the second week of October there. But the rest of the state should have a normal, showy autumn foliage display."

For more than two decades Abrams has studied how precipitation and temperature influence timing and intensity of fall colors. He's found that the critical period for coloration in the Keystone State runs from the middle of September to the second week of October. "At that point, we need nice cool nights in the middle to high 30s -- but not a hard frost -- and bright sunny days," he said.

Ironically, drought in September and early October is generally good for fall colors, but not when the previous July and August is so dry, like this year.

Cooler temperatures signal deciduous trees to stop producing chlorophyll, the green pigment responsible for photosynthesis, Abrams explained. Photosynthesis is the way plants trap light energy and convert it to sugars and starches, the food and building materials for plants.

As the chlorophyll breaks down and disappears, it unmasks other leaf pigments. It's these other pigments -- called xanthophylls and carotenes -- that create the glowing yellows and oranges seen in the leaves of yellow poplar, hickory, sycamore, honey locust, birch, beech and certain maples. After chlorophyll production stops, trees also produce another pigment in their leaves called anthocyanin. The anthocyanins create the brilliant reds seen in red maple, sassafras, sumac and black gum.

"Laboratory and greenhouse research indicates that more anthocyanin is produced when starch levels in the leaves are high," Abrams saod. "Because drought reduces photosynthetic rate -- which in turn decreases starch levels -- people generally believe that drought affects fall coloration in a negative manner.

"But is has to be a very prolonged and severe drought," he says. "We've had a number of summer droughts in the last 20 years, and as long as we get that nice cool-down period, starch levels have been adequate to produce good color."

One thing Abrams has found to be very detrimental to fall colors is a very warm late summer and early fall. "The trees get confused," he says. "They try to stay green longer to capture the longer growing season. This may be good for tree growth, but it's not good for peak coloration.

"One thing to keep in mind -- fall coloration in Pennsylvania forests is very resilient," he added. "Over the range of climatic conditions and challenges we've seen, we've still had good fall colors. This year, if the rains come in the next few weeks, we will probably also get a nice display. "